Growth/differentiation factor 8 (GDF-8), also referred to as myostatin, is a TGF-β family member expressed for the most part in the cells of developing and adult skeletal muscle tissue. Myostatin appears to play an essential role in negatively controlling skeletal muscle growth (McPherron et al. Nature (London) 387, 83-90 (1997)). Mutations in the myostatin gene have been demonstrated in various species, including cattle, pigs, dogs and humans, and have resulted in increased musculature (Kocamis and Killefer, Domestic Animal Endocrinology 23:447; 2002). Moreover, antagonizing myostatin has been shown to increase lean muscle mass in animals (McFerron et al, supra, Zimmers et al, Science 296:1486 (2002)).
Myostatin antagonists have also been evaluated in human clinical trials. A human antibody referred to as MYO-29 was evaluated in patients with various forms of muscular dystrophy. Early clinical results with this myostatin antagonist demonstrated good safety and tolerability, with no noted improvements in muscle strength or function (however, the study was not powered to demonstrate efficacy); a trend toward increased muscle size was noted in a limited number of subjects (Wagner et al. Ann. Neurol. 63:561; 2008). In subsequent reports, overall quantitative muscle strength measurements in treated patients did not improve, however several patients exhibited improvement in single muscle fiber contractile properties (Krivickas et al. Muscle Nerv. 39:3; 2009).
Regulation of the myostatin pathway is believed to require processing of a latent myostatin complex into mature myostatin. The latent complex is formed of a cleaved propeptide domain that is noncovalently associated with a mature C-terminal dimer, and is biologically inactive. Tissue-specific factors are thought to be responsible for converting the inactive complex into the biologically active form. Myostatin also forms a complex with follistatin-related gene (FLRG) and growth and differentiation-associated factor-associated serum protein-1 (GASP-1), both of which complexes have been identified in serum.
Mature myostatin binds with high affinity to the activin type IIB receptor (ActRIIB), and with lesser affinity to the activin receptor (ActRIIA). Intracellular signalling is initiated by binding of dimeric myostatin to ActRIIB followed by recruitment of a low-affinity type I receptor, either activin-like kinase 4 (ALK4) or activin-like kinase 5 (ALK5). Phosphorylation of the type I receptor results in initiation of the intracellular signalling pathway that is responsible for myostatin's biological effects.
Utility of myostatin antagonists in vivo has been complicated not only by the nature of regulation and signalling of the myostatin pathway but also by the high degree of similarity of myostatin to growth and differentiation factor 11 (GDF-11; also known as bone morphogenetic protein 11 or BMP-11), which is 90% identical to myostatin at the amino acid level, in the active domain. While the high degree of sequence identity and similarities in signalling mechanism suggest that myostatin and GDF-11 share certain functions, targeted gene disruptions of these two TGF-beta family members show very different results. Myostatin knockout mice exhibit hyperplasia and hypertrophy of myofibers, and GDF-11 knockout mice die shortly after birth with numerous abnormalities; dual knockout animals show additional abnormalities not seen in single knockout mice (McPherron et al., BMC Dev Biol. 9: 24; 2009).
Accordingly, there is a further need in the art for agents that bind myostatin and antagonize its activity while eliminating or minimizing adverse effects of inhibiting this and related pathways.